The present invention relates to photovoltaic cells, and more particularly to a low cost, high efficiency heterojunction p-i-n photovoltaic cell.
Photovoltaic cells convert visible and near visible light energy to usable direct current electricity. Photovoltaic cells encompass solar cells which convert the visible and near visible light energy of the sun to usable direct current electricity.
The known heterojunction photovoltaic cells utilize two semiconductor materials to produce a rectifying junction. The advantages of utilizing this design include the ability to choose materials with properties appropriate for each component of the device and the reduced necessity for compromise with the property requirements of other components of the device. An example of this is the use of a wide band gap "window" semiconductor material of one carrier type (e.g., an n-type) as a barrier layer on a more narrow band gap "absorber" semiconductor material of the opposite carrier type (e.g., a p-type). The amount of radiation absorbed (and therefore the electrical current generated in the device) increases with decreasing band gap width, while the diffusion potential obtainable within the device (and therefore the electrical voltage generated in the device) increases with band gap width. Thus, the absorber material is chosen to maximize the solar radiation absorbed and afford reasonable diffusion potential, while the window material is chosen to absorb a minimum amount of solar radiation. Further design considerations include consideration of the electrical conductivity, chemical stability, density of bulk and interface electron and hole traps and recombination centers, availability of suitable ohmic contacts, electron and hole (i.e., charge carrier) mobilities, electron and hole lifetimes, discontinuities in the valence and conduction bands at the interface, absorption coefficient, material cost, ease of deposition, chemical or environmental stability, preferred carrier type, and other attributes of semiconductors well known in the photovoltaic art.
The principle of the p-i-n structure involves the creation of a diffusion potential across a relatively wide, high resistivity intrinsic layer. This diffusion potential is generated by the p and n regions on either side of the intrinsic layer. A feature of this structure is that the light is absorbed within the field region, and thus photogenerated positive and negative charge carriers are field assisted toward the p and n regions, respectively. In a variation of this structure the band gap of the p or n semiconductor facing the incident radiation is increased to permit more solar radiation to be absorbed within the intrinsic layer. However, even this innovation falls short of fully exploiting the flexibility of design available to producers of heterojunction photovoltaic cells. The p-i-n structure has been utilized in both amorphous and single crystal devices. Single crystal devices have been eschewed, possibly due to the high cost of single crystal materials and the difficulty of depositing them. P-i-n solar cells have been constructed to amorphous materials, but the carrier mobility and lifetime are low.
In addition, while heterojunction cells have been analyzed using the p-i-n model, no heterojunction devices of three or more layers (i.e., heterojunction p-i-n devices) have been produced heretofore.
Accordingly, it is an object of the present invention to provide a heterojunction p-i-n photovoltaic cell combining the ability to chose materials with properties appropriate for each component of the device with the ability to field assist the photogenerated charge carriers towards their respective regions.
Another object is to provide such a cell in which the cost of manufacturing the same is minimized and the photovoltaic efficiency (i.e., the ratio of electrical power output to radiant power input) is maximized.
A further object is provide such a cell using polycrystalline materials for some, and preferably all, of the semiconductor layers thereof.
It is also an object of the present invention to provide such a cell utilizing materials at each junction which minimize the presence of discontinuities or spikes in the energy band which is designed to carry charge carriers out of the absorber layer.
It is another object to provide such a cell having in particular embodiments a high efficiency level and a high optical transmission level.